1. Field of the Invention
The present invention relates to a microwave dielectric composition used for dielectric resonator, dielectric substrate, dielectric antenna and so on, in the field of mobile radio communication such as cellular phone, and more specifically relates to the new microwave dielectric composition having the excellent properties of quality factor, dielectric constant and temperature coefficient.
2. Prior Art
Recent increment of communication information capacity promotes the use of high frequency band, and the communication by microwave of 3 GHz–30 GHz is developed by remarkable force. The representative is cellular phone, and through miniaturization of high quality circuit components, the cellular phone is miniaturized/weight saved/multifunctionized and it becomes very popular quickly.
Transmitter and wave filter are the circuit elements transmitting and receiving microwave signal, respectively and the important components of communication equipment. Since dielectric ceramics are used as the circuit element which is transmitting and receiving microwave by resonance of microwave, said circuit element is called a microwave dielectric resonator. Today, popularization of cellular phone is caused by the inexpensive high quality microwave dielectric resonator.
Next three kinds of property are required for the microwave dielectric resonator.    (1) Relative dielectric constant (∈r) is large: when a wavelength of microwave in vacuum is λ0, the wavelength λ in said dielectric satisfies the equation λ=λ0/√{square root over ( )}∈r.
Since the size of resonator becomes 1/√{square root over ( )}∈r, it is possible to miniaturize the resonator by using of the dielectric of large ∈r.    (2) Quality factor (Q·f: product of Q and f) is large: energy loss occurs when the microwave passes the dielectric, and this dielectric loss is expressed by tan δ. Since so called Q factor is given by Q=1/tan δ, the dielectric having small dielectric loss has large Q factor. This Q factor depends upon the resonance frequency f, and a relationship that the product of Q and f is constant, namely Q·f=const. is concluded. This Q·f is called quality factor, which is used in evaluation of dielectric loss because high quality dielectric has large value of Q·f.    (3) Temperature coefficient of resonance frequency (τf) is almost zero: it is desirable that the temperature coefficient (τf) of resonance frequency of a circuit system is zero or close to zero so that the resonance frequency does not vary with temperature.
Material development satisfying these conditions is going on, but the matter of fact is that the material development of dielectric possessing all three conditions is really difficult. Therefore, conventionally, materials having high dielectric constant value are used in 1 GHz zone such as cellular phone or personal handyphone system (PHS), and composite perovskite materials of large Q factor are used in 10 GHz zone such as down converter of satellite broadcast. In other words, it is the present situation that materials are used selectively according to frequency band.
The present inventors had studied zealously to develop a high quality microwave dielectric composition, and as a result, the inventors had conceived tungsten bronze type compositions mainly consisting of BaO.R2O3.4TiO2. This microwave dielectric composition is expressed by Ba6−3xR8+2xTi18O54 (R is a rare earth element, x satisfies a range of 0.5≦x≦0.7), and already published as Japanese Patent Open-Laid Publication No. 10-274005.
This tungsten bronze type composition is manufactured easily comparatively and suitable in the field of cellular phone requiring as mass production for a short term. However, so far as judging from three conditions of relative dielectric constant ∈r, quality factor Q·f and temperature coefficient τf, the present is the developmental stage of implementation of microwave dielectric material with better quality.
In such situation, the present inventors discovered a series of compositions having excellent microwave dielectric properties in the vicinity of tungsten bronze type composition. It was found that the composition formula of this composition is expressed by BanLa4Ti3+nO12+3n (n=1,2 or 4).
While a further study, the present inventors found that this composition had been already announced as “Microwave Dielectric Properties of Hexagonal Perovskites”(Materials Research Bulletin, Vol. 31(1996)pp. 431–437) by C. Vineis, P. K. Davies, T. Negas and S. Bell.
In this paper, C. Vineis et al. disclose the dielectric properties of composition expressed by composition formula of BaLa4Ti4O15 and Ba2La4Ti5O18. These two compositions are understood as the cases of n=1 and 2 of said series of ceramic composition expressed by BanLa4Ti3+nO12+3n which present inventors systematically discovered. A composition expressed by the general formula with n satisfying 1, 2 or 4 is called homologous composition.
As C. Vineis et al. published, this series of homologous composition is the microwave dielectric material, which satisfies these conditions of the relative dielectric constant ∈r, the quality factor Q·f and the temperature coefficient of resonance frequency τf whose the performance is applicable to microwave products such as cellular phone.
The homologous composition having excellent quality is requested by further development that is not limited to Ba—La—Ti system of the homologous composition expressed by BanLa4Ti3+nO12+3n (n=1,2,4) having excellent characteristic. However, at present, this composition is a pure state ceramic cmposition which is limited to the case of BanLa4Ti3+nO12+3n, and n is limited to integer number of 1, 2 or 4. In the case using only this kind of homologous composition, the range of material selection is narrow and limitation appears in quality improvement and diversification of products.
In addition, as described above, the strict manufacturing conditions relating to ingredient composition, component ratio and purity make degradation of microwave dielectric unit price difficult, and induce the price rise of microwave dielectric resonator. Therefore, the study to realize the reduction of manufacturing price of microwave dielectric becomes definitely necessary for the further spread of cellular phone and its excellent performance.
Therefore, the first object of the present invention is to develop the microwave dielectric composite composition having wide variety and excellent quality by adding other elements to Ba, La and Ti with keeping homologous structure expressed by BanLa4Ti3+nO12+3n. In addition, the second object of the present invention is to realize the microwave dielectric composite composition having high quality and low price by expanding the index n to vicinity of 1, 2 and 4.